The present invention is directed toward oval gears for fluid metering devices and more particularly to gear tooth geometry for oval gears. Hand-held devices are often used to dispense measured amounts of fluid from bulk containers. For example, automotive service stations frequently use hand-held meters to dispense small quantities of lubricating oil from large drums into automotive engines. Such hand-held meters and other similar fluid dispensing devices typically include a positive-displacement metering mechanism that measures volumetric flow of fluid passing through the dispensing device. Conventional positive-displacement metering mechanisms include a set of intermeshing elliptical or modified-elliptical oval gears, between which a pressurized fluid flows to cause rotation of the gears. The gears are typically connected to an electronic control system that counts the revolutions of the gears to determine the flow volume of the fluid. The gears are journaled such that the distance between the centers of the gears is fixed. The gear teeth are thus brought into engagement along segments of the gear pitch curves having different curvatures, which introduces complexities in the interface of the gear teeth. Performance of the dispensing device is, however, related to the effectiveness of the gear tooth interface between the gears. For example, the accuracy with which the metering mechanism is able to determine flow depends on the ability of the tooth interface to seal and prevent fluid leakage between the gears that does not contribute to rotation of the gears. Additionally, tooth interface affects the longevity of the life of the gears, the amount of noise produced by the gears, and the amount of vibration produced by the gears; all of which depends on the ability of the gear teeth to smoothly roll against each other. Thus, gear tooth design is important to effective metering devices and accurate dispensing of fluid.
Conventional circular spur gears often use gear teeth having involute gear tooth surfaces, which tend to roll across each other as the gears rotate, rather than sliding and chattering against each other. Involute curves, as are conventionally known, can be described as the path an end of an inextensible cord travels as it is unwound from a curved surface, such as an oval or a circle. The base profile of the gear is typically used as the curved surface for forming the involute gear tooth profile. Thus, circular spur gears have circular base curve profiles that result in circular involute tooth profiles. For circular gears, the involute surfaces are the same on each tooth and the pitch radius of the gear is the same for each tooth. As such, involute gear teeth are easily produced for circular gears, such as with conventional hobbing machines and the like. A typical hobbing process involves rotating a hobbing cutter, a cylindrical cutting tool having helical cutting teeth with the reverse profile of the gear teeth, against a rotating gear blank. Because of the changing angle of incidence between oval gears, it is impractical to use conventional hobbing processes to produce involute gear teeth for oval gears. For example, an oval gear blank would need to be translated perpendicularly toward and away from the hobbing rack at intervals corresponding to the changing radius of the oval. Even with the ability to translate the gear blank, the gear teeth would have a tendency to be undercut because of the elliptical pitch curve. Various gears having modified tooth geometries and/or pitch profiles have been designed to produce smoothly interfacing oval gears that are more easily manufactured.
Early gear designs approximated the shape of the tooth geometries and the pitch curve to achieve smoothly interfacing gears. U.S. Pat. No. 231,939 describes a modified elliptical gear in which the pitch curve is closer to the center of the gear near the minor axis to form a lobe-shaped gear. The profiles of the gear teeth are approximated by dividing the pitch curve into segments and producing a series of small arcs intersecting the pitch line to produce a smooth rolling shape that does not have an involute profile. In subsequent designs, gear tooth sizes were changed to achieve actual elliptical involute tooth profiles on elliptical gears and to facilitate manufacture with hobbing machines. For example, U.S. Pat. No. 2,842,977 describes an elliptical gear having gear teeth with surfaces that are involutes of an elliptical pitch curve that can be produced with a hobbing machine. However, in order to avoid the need for translating the blank perpendicularly with the hobbing rack, the size of the gear teeth increases from the major axis to the minor axis, thus requiring a specially designed hobbing cutter. Furthermore, elliptical gears are not suitable for use in fluid metering devices, as the centers between rotating elliptical gears cannot be fixed. In other designs, both the pitch curve and the tooth geometry have been modified to achieve improved gear tooth interaction. In U.S. Pat. No. 2,897,765 gears are produced having a modified elliptical pitch curve in which the pitch curve is closer to the center of the gear near the major and minor axes as compared to a true ellipse to relieve gearing pressure. The gear teeth are, however, wider and taller near the major axis than near the minor axis to increase the strength of the teeth and reduce the number of teeth.
As technology has progressed, more elaborate gear tooth profiles and pitch curves have evolved to produce elliptical involute teeth. For example, U.S. Pat. No. 4,036,073 describes an elliptical gear in which the gear teeth have varying tool pressure angles such that the gear can be produced with a hobbing mechanism that translates the gear blank along two axes relative to the hobbing rack. However, in addition to requiring an elaborate hobbing machine, the teeth at and near the major and minor axes have wider tips to avoid undercutting. U.S. Pat. No. 5,545,871 describes a modified elliptical gear in which the pitch curve is bulged at portions of the curve between the major and minor axes relative to a true elliptical gear. Using a computer design system, a simulated hobbing process is used to produce elliptical involute shaped teeth based on a smaller scale version of the modified elliptical pitch curve.
Finally, other gear designs have evolved that avoid the use of involute teeth in attempts to achieve improved gear performance. In U.S. Pat. No. 6,048,186, gears are produced having a modified elliptical pitch curve in which the pitch curve is closer to the center of the gear near the major and minor axes as compared to a true ellipse. The gear teeth have an involute shaped surface facing the major axis and a cycloid gear shaped surface facing the minor axis to prevent problems associated with trapping and to prevent the gears from coming out of mesh. Still, other designs avoid the use of involute gear teeth altogether. U.S. Pat. No. 6,644,947 discourages the use of involute gear teeth and describes an oval gear in which the gear teeth have a “wave tooth” pattern. The heads of the teeth are shaped by an arc having a first radius and the roots of the teeth are formed by an arc having a second radius. This design results in flat gear tooth surfaces that reduces sliding, but are neither elliptical nor circular involutes.
The foregoing gear designs illustrate the importance of gear tooth interface in avoiding performance problems in fluid metering devices such as undercutting of the tooth root, scoring of the tooth face, resistance to rotation from trapping of fluid between teeth, leakage of fluid through meshed teeth and the like. Such designs, however, are typically tradeoffs between one or more of the performance parameters for the gear tooth interface and a manufacturing parameter. For example, non-involute gear teeth may be easier to manufacture, but result in relative angular velocities between gear teeth, which produces sliding and chatter that result in tooth wear and noise. There is, therefore, a need for an improved gear tooth design for oval gears, particularly those used in fluid metering devices.